The present invention relates to a detector assembly for a panoramic or sectorial infrared monitoring system.
Such a detector assembly may comprise an optical device, in the focal plane thereof, a detector strip--infrared photodiodes--for analyzing landscapes, or the background of a scene, hybrided to a pre-processing wafer containing charge transfer circuits, namely and essentially, input circuits associated with the photodiodes, for integrating their output currents, preamplification, filtering and multiplexing circuits and an output circuit forming an interface between the preprocessing wafer and a separate subsequent processing wafer.
When the analysis strip comprises photodiodes disposed in a plurality of lines of several photodiodes, the output current of each photodiode may then be processed in an integration circuit delivering a charge signal to a delay line in which it undergoes a given phase-shift, the charge signals delivered by all the delay lines associated with the photodiodes of the same line being added in a summator. The circuit in question is a delay-summation circuit called TDI (time delay integration).
The pre-processing wafer may also comprise base-clipping, evacuation and anti-dazzle circuits, the purpose of the latter function being to derive all the diode current when it is too high and to prevent a saturating current from adversely affecting the operation of the circuits of the processing wafer.
Infrared detector assemblies are used at the present time either in monitoring systems, or in image formation systems or in combined systems.
In image formation systems, for analyzing landscapes in real time, the elementary detectors of the analysis strip must be the smallest possible so as to obtain the best possible angular resolution and thus optimize the recognition and identification range performances.
The strips of the image formation systems may be of several types, for example and at least, the four following ones:
1. Strip comprising a series of elementary detectors aligned in columns and slightly offset with respect to each other. Such an arrangement involves interlacing of the scanning frames, i.e. a shift from one frame to the next, by an opto-mechanical device, which however leads to only a low overlapping rate.
2. Strip comprising two series of elementary detectors aligned respectively in two columns with, in each column, a shift between two adjacent detectors by the dimension of a detector, the shifts of the two columns being alternate. Such a geometrical arrangement also involves interlacing of the scanning frames to obtain an overlap rate close to 2.
3. Strip comprising more than two series, for example four, of detectors aligned respectively in as many columns with, in each column, between two adjacent detectors, the same shift as in the strip of the second type, the respective shifts of the columns being themselves slightly offset with respect to each other. Such geometry already has an overlap rate close to 2 and therefore does not require frame interlacing.
4. Strip comprising two mosaics of detectors disposed in m lines and n (for example four) columns, the respective lines of the two mosaics being alternate. This geometry, similar to that of type 2 provides a redundancy, each image element being analyzed successively by n detector elements.
In monitoring systems the elementary detectors of the strips used up to now were on the contrary sufficiently large to adapt themselves to the optical spots produced by the hot points of the objects (aircraft and other targets) to be detected and whose temperature is very much greater than that of the background of the scene.
As an example of monitoring strip geometry, two columns of detectors have been adopted spaced apart by the dimension of an optical spot produced by a pinpoint target and the detectors of the two columns being respectively slightly offset so as to obtain a certain overlap from one column to the next in order not to produce a signal loss and so to make the target extraction algorithms efficient.
The monitoring strips used up to now, with detectors having a relatively large sensitive area, have however two major drawbacks.
Produced on a small scale, their cost is first of all very high. Then, they do not use redundancy. With a redundancy of order n provided by an IR-CCD detector, saturation of the storage capacities and the processing circuits is rapidly reached because of the size of the elements and so of the amount of photons collected. Technology does not allow the dynamics of the processing circuits to be extended at will. It is necessarily limited, which inevitably leads to a saturation phenomenon which is quite prejudicial in monitoring in which it is desirable to detect objects whose temperature is very much greater than that of the background of the scene. For example, in the saturating temperature range, two hot points of respectively different temperatures can no longer be differentiated. The larger the sensitive areas of the detectors, the higher the fixed spatial noise, the higher should be the dynamics also and, since this is not so, the more the processing circuits risk being saturated.
The purpose of the present invention is then to overcome these drawbacks.